Enhanced radiative transfer and heat recovery for combustion device

ABSTRACT

A stove includes a combustion chamber for producing heat, a thermoelectric device thermally coupled to a hot source and a cold source, a battery, a fan electrically connected to the thermoelectric device and the battery, and a controller configured to monitor the battery and thermoelectric device and configured to direct the fan be operated from power provided from the battery when power produced from the thermoelectric device is insufficient to power the fan. The stove can have an associated heating plate, and the walls of the combustion chamber can be configured to reflect heat onto the heating plate.

BACKGROUND

The ability to generate power in remote locations finds many uses formilitary, civilian, and commercial applications. For military uses,portable power is required for any forward or remote operating basesthat cannot tie into an existing power supply grid. Portable power alsois useful in civilian applications, for example, during natural disasterrelief operations. Portable power is necessary for operating machineryto assist in the disaster relief efforts, such as communications and forillumination, or to provide for basic needs of the population, such asrefrigeration of foods and cooking. Portable power also has uses in thecommercial realm. For example, portable power is useful when camping orfor use when traveling to any location that is not serviced by anexisting power grid. Additionally, many residences experience poweroutages during severe storms, and having backup power sources would bevery desirable. One form of power generation relies on liquid fuels topower generators. In some cases, liquid fuels may be unavailable orextremely difficult to obtain. In a mobile application, the liquid fuelalso needs to be transported. Biomass, such as wood and othercombustibles, provides an alternative to liquid fuels that has theadvantage that it may be present on site.

Converting biomass into power has inherent problems. Biomass, forexample, can be difficult to combust initially and will take a long timeto heat up using natural convection. Accordingly, forced air draftsystems could be used, but there needs to be a reliable method ofproviding forced air flow that could speed the system startup. Also,biomass can be inefficient because it generally produces lowtemperatures. Accordingly, there needs to be a means for increasing theefficiency of biomass-burning stoves.

Disclosed herein is a stove for power generation that may address one ormore of these deficiencies.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect of the present invention, a stove has a combustion chamberand a heating plate, the combustion chamber having one or more angledcombustion chamber walls flared relative to each other so as to beconfigured to reflect heat produced in the combustion chamber onto theheating plate. The walls can define a combustion chamber of V-shape,narrowed at the bottom and widened at the top in the area of the heatingplate.

In other aspects of the invention, one or more combustion air inletducts are formed from a side of one of the flared or angled combustionchamber walls.

In other aspects of the invention, a recuperator is located at the baseof the stove, the recuperator being thermally coupled to the combustiongas exhaust ducts and to the air inlet ducts.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration showing one embodiment of a stoveaccording to the present invention;

FIG. 2 is a schematic illustration showing the elements of a powermanagement and distribution system for the stove of FIG. 1;

FIG. 3 is a step logic diagram for a method of operating the stove ofFIG. 1;

FIG. 4 is a diagrammatical illustration of a combustion device withradiative heat transfer and heat recovery;

FIG. 5 is a diagrammatic illustration of a recuperator that can be usedwith the combustion device of FIG. 4; and

FIG. 6 is a diagrammatic illustration of the recuperator of FIG. 5,viewed from the right of FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1, a portable power generating stove 100, inaccordance with one embodiment of the invention, is illustrated. Thestove 100 includes a combustion chamber 126 suitable to burn biomass,including, but not limited to solid fuels, such as wood, paper,cardboard, plant matter, animal dung, pellets, waste, trash, tires, andother combustibles. Alternatively, the stove 100 may be modified toaccept liquid or gas fuels. For example, the stove 100 may be equippedwith a suitable burner to burn liquid fuel, or gas fuel. In oneembodiment, a combination of solid, liquid and/or gas fuels may be usedsimultaneously by providing a plurality of combustion chambers. Forexample, one combustion chamber may be used for burning of solid fuel,and a second combustion chamber may be used for burning liquid fuel.

The combustion chamber 126 and stove 100 may be made generally frommetals, with an insulating material between the combustion chamber 126and stove exterior 102. The stove 100 includes a heating plate 106 (orheat acceptor plate) positioned directly above the combustion chamber126. The heating plate 106 is heated mainly by radiative and convectiveheating from combustion and combustion gas products impinging on theheating plate 106. The stove 100 includes a Stirling engine 104, suchthat the heating plate 106 is thermally coupled to the Stirling engine104, either directly or through one or more heat exchangers. Thespecific design of the Stirling engine 104 will dictate the manner oftransferring heat from the heating plate 106 to the Stirling engine. Asused herein, a Stirling engine is a well known device that operates bycyclical compression and expansion of a working fluid, such as air orother gases. The Stirling engine 104 compresses and expands the workingfluid by cyclical cooling and heating of the working fluid using a heatsource and a heat sink (cold source). The result is a net conversion ofheat energy to mechanical work. In the stove 100, the heating plate 106is used as the heat source for the Stirling engine, thus, powering theStirling engine, which in turn may power a generator. However, in analternative embodiment, the hot end of the Stirling engine 104 may bewithin the combustion chamber 126.

A simple Stirling engine may use a single cylinder (beta-typeconfiguration) with a hot end and a cold end or two cylinders(alpha-type configuration), where one cylinder is exposed to the heatsource and the second cylinder is exposed to the cold source. Morecomplex Stirling engines may aggregate either one or a combination ofthe simple one or two cylinder designs into a multiplicity of cylindersand complex piston arrangements. A Stirling engine is classified,similar to a steam engine, as an external combustion engine, as heattransfer between the combustion gas and the working fluid occurs throughthe cylinder wall and no combustion takes place inside of the cylinder.While any Stirling engine may be used as the Stirling engine of thepresent invention, a suitable Stirling engine is disclosed in U.S. Pat.No. 7,134,279, to White et al., which is fully incorporated hereinexpressly by reference. This patent discloses a double-acting,multi-cylinder, thermodynamically resonant, alpha configurationfree-piston Stirling system. The system includes overstroke preventersthat control the extent of piston travel to prevent undesirableconsequences of piston travel beyond predetermined limits. Theoverstroke preventers involve controlled work extraction out of thesystem or controlled work input into the system. The patent disclosesthat the Stirling engine may be coupled for electrical power generationin which alternating current (AC) power output can be rectified andfiltered to provide direct current (DC) power, and that three phase ACpower output from a three cylinder module implementation can beconverted to DC power with good efficiencies and simple electronics.

The stove 100 includes a flue gas duct 114 for the combustion gasesgenerated in the combustion chamber 126. The stove 100 includes athermoelectric device 122 placed at a location in proximity or incontact with the flue gas duct 114. The stove 100 includes an air inletduct 116. The thermoelectric device 122 is also in proximity or incontact with the inlet air duct 116. The stove 100 includes a fan 118provided at the inlet of the air inlet duct 116. The fan 118 providesforced draft combustion air to the stove 100. The air is forced into airducts 124 that lead into the combustion chamber 126. The Stirling engine104 will be able to start faster and reach operating temperatures fasterwith forced draft combustion air provided by the fan 118.

The thermoelectric device 122 is a well known device. For example, U.S.Pat. No. 7,942,010, to Bell et al., which is fully incorporated hereinexpressly by reference, discloses thermoelectric modules can be used forpower generation. In FIG. 27, for example, Bell et al. discloses adesign using thermoelectric modules for generating power. Thethermoelectric device 122 generates power based on a temperaturegradient. In the embodiments of the instant invention, the differencebetween the hot flue gas duct 114 and the cold air inlet duct 116 canprovide the temperature gradient used by the thermoelectric device 122The thermoelectric device 122 may include a plurality of modules, eachone having a P-type and N-type semiconductor. Increasing the number ofmodules will increase the power output of the thermoelectric device 122for any given temperature difference. The number of modules will dependon the desired power output from the thermoelectric device 122. Each ofthe P-type conductors and each of the N-type conductors can bealternately electrically connected to each other in series withelectrical conducting shunt elements, which may also serve as thermalconducting shunts from both the hot and cold source, i.e., the hot fluegas duct 114 and the cold air inlet duct 116. However, the shunts shouldbe electrically isolated from the hot flue gas duct 114 and the cold airinlet duct 116. This arrangement can be used to provide a voltagedifferential, which supplies current to power a load.

The hot flue gas duct 114 passes through a recuperator 120 (i.e., a heatexchanger). The recuperator 120 is also in thermal contact with the airinlet duct 116. The recuperator 120 allows heat to be transferred fromthe combustion gases to the incoming air, thus preheating the air beforecombustion and providing for more efficient use of the fuel.Recuperators are well known devices for transferring heat from one gasto another and may utilize fin and tube designs, single-pass,multi-pass, and countercurrent flow patterns. The flue gas is hotter onthe hot side before the recuperator 120 than on the cold side after therecuperator 120, and the air will be colder on the cold side before therecuperator 120 than on the hot side after the recuperator 120.Accordingly, this provides several options for thermally coupling thethermoelectric device 122 to provide a temperature differential. In oneembodiment, the thermoelectric device 122 can be coupled to the hot sideof the hot flue gas duct 114 and the hot side of the cold air inlet duct116 (this is illustrated in FIG. 1). In one embodiment, thethermoelectric device 122 can be coupled to the hot side of the hot fluegas duct 114 and the cold side of the cold air inlet duct 116. In oneembodiment, the thermoelectric device 122 can be coupled to the coldside of the hot flue gas duct 114 and the hot side cold air inlet duct116. In one embodiment, the thermoelectric device 122 can be coupled tothe cold side of the hot flue gas duct 114 and the cold side cold airinlet duct 116. In other embodiments, the thermoelectric device 122 canbe coupled to hot surfaces of the stove 100 as the hot source andambient air serves as the cold source for the thermoelectric device 122.When the thermoelectric device 122 is placed away from the recuperator120, such as the outer surface of the stove 100, the performance of therecuperator 120 can be increased due to the removal of thethermoelectric heat path.

The stove 100 includes a plurality of temperature measuring devices 108,110, and 112. The temperature measuring device 108 may measure thetemperature of the hot end of the Stirling engine 104. The temperaturemeasuring device 110 may measure the temperature of the heating plate106. The temperature measuring device 112 may measure the temperatureinside the combustion chamber 126. The temperature measuring devices108, 110, and 122 are well known devices, such as thermocouples, ratedto withstand temperatures in excess of approximately 1000° C. Whilerepresentative locations of the temperature measuring devices 108, 110,and 112 have been illustrated, it is to be appreciated that theselocations are not limiting, and more or less temperature measuringdevices may be used, such as on either the hot or cold side of the hotflue gas duct 114 and the hot or cold side of the cold air inlet duct116. These temperature measuring devices may be used in various controlalgorithms as further described below.

FIG. 2 is a schematic illustration showing the elements of a powermanagement and distribution (PMAD) system used for the stove 100. Inaddition to the features discussed in association with FIG. 1, the stove100 includes a battery 206, a resistor 204, a controller 202, and powerconnection and communications cables connecting the various elements.The elements of the power management and distribution system include thecontroller 202, the thermoelectric device 122, temperature sensingdevices 108, 110, 112, a battery 206, a fan 118, and a resistor 204. Thecontroller 202 is any well known central processing unit that may beused to perform a series of logic decisions based on several inputsreceived from the system elements. The thermoelectric device 122provides power to the system including the controller 202 and fan 118.However, as mentioned above, the thermoelectric device 122 relies on atemperature gradient being produced between the hot flue gas duct 114and the cold air inlet duct 116. Accordingly, during stove 100 startupand insufficient temperature gradient conditions, the battery 206 isprovided for start of the system. The battery 206 can be any type ofrechargeable energy storage device, including but not limited to leadacid batteries, liquid electrolyte batteries, gel batteries, absorbedglass mat batteries, and dry batteries, such as nickel cadmium, nickelzinc, nickel metal hydride, and lithium ion. The battery 206 may includeinstruments for determining the state of charge of the battery. State ofcharge can be calculated by measuring any one of several parameters ofthe battery including the electrolyte specific gravity, voltage,current, and temperature. This information is used by the controller 202for determining the state of charge of the battery 206 and makingdecisions whether to power the fan 118 from the battery 206 or from thethermoelectric device 122. In one embodiment, if battery state of chargeindicates that the battery 206 is charged to capacity, and thethermoelectric device 122 is producing more power than what is beingconsumed, the excess power generated by the thermoelectric device 122may be shunted to the resistor 204, where the power is dissipated asheat. However, in the normal operating mode of the stove 100, thebattery 206 is being charged by the thermoelectric device 122 and thefan 118 is being powered by the thermoelectric device 122. In otherembodiments, the resistor can be any load, such as an electrical load,including, but not limited to radios, lights, heaters, and the like.

The system includes a fan 118, which is capable of being provided withpower from the battery 206 as well as from the thermoelectric device122. The fan 118 may be a variable speed fan which bases its speed onone or more of the stove temperatures 108, 110, and 112. The controller202 receives the temperature measurements, and may make decisionswhether to run the fan faster or slower, or stop the fan altogether.Generally, if a decision is made by the controller 202 that atemperature of interest needs to be higher, the fan 118 speed isincreased to provide higher burn temperatures. For example, the Stirlingengine 104 may operate most efficiently if a certain temperature isachieved. As the stove 100 burns hotter, the fan 118 speed is increased,thereby providing more combustion air. The controller 202 can receivethe stove temperatures 108, 110, and 112, and set a corresponding fan118 speed. The controller 202 also directs which power source is used topower the fan 118 depending on battery 206 state of charge and thethermoelectric device 122. When a temperature gradient is produced andthe controller 202 sensing that the power from the thermoelectric device122 is sufficient, the controller 202 may direct that power generatedfrom the thermoelectric device 122 is used to power the fan 118. Forexample, a temperature differential between the hot and cold source ofthe thermoelectric device 122, a hot temperature, or a voltage may beused to determine when the thermoelectric device 122 is producing therequired power. If the controller 202 senses that the thermoelectricdevice 122 is producing more power than what the fan 118 and system as awhole require, the controller 202 may direct that some of the powerproduced by the thermoelectric device 122 be used to charge the battery206. For example, when the controller 202 senses that a temperature hasreached a lower limit, the controller 202 assumes that thethermoelectric device 202 is producing sufficient power and closes aswitch to connect the thermoelectric device 122 to charge the batteryand power the fan. In other embodiments, the fan 118 can simply beconnected directly to the battery 206, and the thermoelectric device 122supplies power to the battery 206. If the controller 202 decides thatthe thermoelectric device 122 is producing too much power because thebattery 206 is fully charged and the fan 118 load is low, either throughactual measurements of the battery 206 and fan 118, or through inferencefrom a temperature measurement, the controller 202 may direct that aswitch be closed so that the thermoelectric device 122 can dump itspower to the resistor 204. The system may have protections to avoidexcessive temperature that may damage one or more components. If a hightemperature condition is detected, the controller 202 may direct thatthe fan 118 speed be reduced or stopped altogether if the hightemperature condition has not cleared for more than a specified periodof time. In addition to reducing fan load, the thermoelectric device 122power can be dumped to the resistor 204.

The controller 202 may also sense an overspeed or overtemperaturecondition. When an overspeed or an overtemperature condition isdetected, the extra power being generated by the thermoelectric device122 can be dumped to the battery 206 to charge the battery 206 or to theresistor 204 to protect the thermoelectric device 122. A high operatingtemperature of the stove 100 means that a high delta-T zone allows forless expensive thermoelectric devices to be used. The linking of the fan118 to flow air past the thermoelectric device 122 allows for increasedheat dissipation. The system can start even when the battery 206 isdead. In this condition, the stove 100 can start on natural convection,and soon combustion gases will provide the temperature differential toallow the thermoelectric device 122 to generate electricity for the fan118, which will then assist in startup and lead to a decrease in thestartup time.

FIG. 3 is a step logic diagram showing one embodiment of a method 300for starting the stove 100.

In block 304, the controller 202 can determine the state of charge ofthe battery 206 and based on the state of charge, the controller 202makes a determination whether the fan 118 can be powered by the battery206. If the determination in block 304 is YES, the method enters block306, where the controller 202 can turn on the fan 118 supplied withpower from the battery 206, and the stove 100 is started using forceddraft air for a faster startup of the stove 100. The fan 118 may have astartup mode designating a proper speed for starting the stove 100.

If the determination in block 304 is NO, the method enters block 310. Inblock 310, the controller 202 does not turn the fan on, and thethermoelectric device 122 is still not producing power. In this case,the user can start the stove 100 using only natural convection. Once thetemperature differential rises to a predetermined value, the controller202 may determine that the thermoelectric device 122 is producingsufficient power and the controller 202 may determine to start the fan118 on power produced by the thermoelectric device 122. The controller202 may rely on a temperature sensed in the combustion chamber 126 or atemperature differential between a hot and cold source coupled to thethermoelectric device 122. Alternatively, the controller may sense avoltage produced by the thermoelectric device 122 to decide when toallow startup of the fan 118.

From block 310, the method enters block 312. In block 312, thecontroller 202 makes a determination whether the thermoelectric device122 is producing more power than consumed. In making this determination,the controller 202 may receive inputs of various instruments. Forexample, the controller 202 may receive the amperage draw from the fan118, and the amperage supplied by the thermoelectric device 122. Todetermine whether the thermoelectric device 122 is producing more powerthat consumed, the controller 202 may receive the amperage from an ampmeter that senses the amperage produced by the thermoelectric device122, and the controller senses the amperage required for the fan 118 tooperate. The fan amperage may be predetermined and stored in acorrelation table that correlates a fan speed to a fan amperage. Thecontroller may also sense the thermoelectric device 122 voltage via avoltage meter, and the controller 202 may determine whether thethermoelectric device 122 is producing excess power through calculationsbased on the amperage and/or voltage. Additionally, in some embodiments,the controller 202 senses the state of charge of the battery 206 inmaking the determination whether or not there is excess power producedby thermoelectric device 122, In another embodiment, the powergeneration of the thermoelectric device 112 can be provided in a tablestored in a memory device within the controller 202. The table includesa correlation of the temperature difference between the hot source andthe cold source correlated to a power being supplied by thethermoelectric device 122. Based on measurements such as these, thecontroller 202 may determine whether the system power requirementsexceed the power generation of the thermoelectric device 122.

From block 306, the method enters block 308. The determination of block308 is similar to the determination made in block 312.

If the determination in either of block 308 and block 312 is NO, themethod continues to operate the fan 122 and all other power loads of thesystem using the thermoelectric device 122. However, if thedetermination in block 308 or block 312 is YES, signifying that thethermoelectric device 122 is producing more power than is consumed, themethod enters block 314. In block 314, the controller 202 directs thatthe excess power from the thermoelectric device 122 be used to chargethe battery 206 while power from the thermoelectric device is also usedto power the fan 118.

From block 314, the method enters block 316. In block 316, thecontroller 202 can determine whether or not the battery 206 is fullycharged. Depending on the type of battery, the controller 202 may usethe specific gravity of electrolyte, the voltage of the battery, theamperage of the battery, and the temperature of the battery to determinethe state of charge of the battery 206. If the determination in block316 is NO, the controller 202 continues to allow charging of the battery206. However, if the determination in block 316 is YES, the methodenters block 318. In block 318, any excess power not consumed by the fan118 is dissipated through the resistor 204.

Referring back to FIG. 1, heat transfer from the combustion chamber 126to the heating plate 106 can be via radiation, conduction, andconvection. The purpose of the heating plate 106 is to act as a means tocollect heat to be used in a power conversion device, such as theStirling engine. Heat transfer is almost minimal through conduction,which would involve heating of the sidewalls of the combustion chamberthat touch or contact the heating plate 106. As between radiative heattransfer and convective heat transfer, it has been discovered by theinventors that radiative heat transfer can be more efficient thanconvective heat transfer. This is because convective heat transfer leadsto a large recuperator (heat exchanger) that would typically be locatedat the top of the stove. From a manufacturing standpoint, this canpresent a packaging issue, since a large area or volume would berequired in order to effectively recover the heat in the recuperator.

In accordance with another embodiment of the invention, a combustionchamber design is provided that increases the radiative heat transferfrom a combustion chamber to a heating plate. This combustion chamberdesign may be used with the stove 100 illustrated in FIG. 1.

Referring to FIG. 4, a schematic illustration is provided of a stove 400with a combustion chamber 404 having angled walls 418 that form aV-shaped combustion chamber 404. The stove 400 may include all thefeatures and operate similar to the stove 100 described in associationwith FIGS. 1-3. The differences between the stove 100 of FIG. 1 and thestove 400 of FIG. 4 will be apparent from the description that follows.

The stove 400 includes a heating plate 410 located directly above thecombustion chamber 404. The angled walls 418 increase the view angle ofthe heating plate 410 and additionally reflect heat from the walls 418and direct it to the heating plate 410. The flame radiates heat 414upward so that it directly impinges on the heating plate 410.Additionally, the flame radiates sideways heat 416 which is reflected bythe angled walls 418 toward the heating plate 410. The angled walls 418may be provided on all sides of the combustion chamber 440, or theangled walls 418 may be provided on at least two opposing sides of thecombustion chamber 404. The angled walls 418 may be made of metal, suchas cast iron.

Combustion gases 408 may exit the stove 400 through a hot flue gas duct420, provided on one or both sides of the heating plate 410. Forceddraft combustion air 406 may enter the combustion chamber 404 throughair ducts 422. Ducts 422 may be provided on one, both or all sides ofthe combustion chamber 404. Ducts 422 may be formed using the oppositeside (or surface) of the angled walls 418 facing the combustion chamber.

The angled combustion walls 418 increase the view factor of the heatingplate 410. “View factor” is the fraction of all the radiative heat fromthe flame that strikes the surface of the heating plate, including thereflected heat. The angle and the length of the combustion chamber wallsare determined to achieve the greatest amount of reflection toward theheating plate 410 depending on the dimensions of the combustion chamber,and the height and length of the heating plate 410. The angledcombustion chamber walls 418 provide for an increase in reflection ofheat 416 to the heating plate 410. Additionally, the angled combustionchamber walls 418 allow for the fire box to be swapped out with aburner. A further advantage of the angled combustion chamber walls 418is that ash will preferentially fall to the bottom of the combustionchamber 414, thus simplifying the collection and the removal of ash fromthe combustion chamber 404.

The heating plate 410 may be coupled with any power conversion orchemical process 412 for use in power generation. As described above,one power generating device can be a Stirling engine. In one embodiment,the Stirling engine 104 can operate more efficiently with a higher hotend temperature, such as 850° C. Accordingly, the heating plate 410should be suitable to withstand temperatures in excess of 850° C. TheV-shaped combustion chamber 404 as well as the use of force draftcombustion air will be able to achieve such temperatures.

A recuperator 402 can transfer heat from the combustion gases 408 to thecombustion air 406, thus increasing performance. In one embodiment, therecuperator 402 is placed around the base of the combustion chamber 404.Locating the recuperator 402 around the stove 400 base leads to betterstability due to a wider base and increased mass on the lower section ofthe stove 400. The combustion gases 408 can be ducted through therecuperator 402 located around the base of the V-shaped combustionchamber 404.

Referring to FIG. 5, a diagrammatical illustration of one embodiment ofa recuperator 402 is shown for the stove illustrated in FIG. 4, FIG. 5illustrates one side of the recuperator 402. However, it is to beappreciated that the opposite side may be constructed similarly.Furthermore, the stove 400 of FIG. 4 may have a recuperator 402 on one,two, three, or all four sides. The recuperator 402 is shown next to thetriangular shaped combustion chamber 404

One representative embodiment of a recuperator 402 includes a shell andtube flow heat exchanger. However, in other embodiments, a recuperatorcan be a series of flat plates with fins in the gas stream to increaseheat transfer. The gas can be arranged to flow countercurrent withrespect to the incoming air to the combustion chamber. Countercurrentflow means the hot flue gas and the incoming air flow from oppositedirections. In other embodiments, the hot flue gas and air flow is in across flow configuration, where flows are 90° offset from each other. Instill other embodiments, the flow can be cocurrent where the flow is thesame direction for the hot flue gas and the incoming air. Recuperatorscan have means to increase the amount of surface area to provide forgreater amount of heat transfer. For example, fins can be included inany square, triangular, or other type of geometry.

The recuperator 402 includes an inlet duct 432 for incoming air 406. Theduct 432 is fitted with a fan to provide forced air combustion for thestove. The fan may be controlled similarly to the fan 118 shown anddescribed in association with FIGS. 1-3. The incoming air duct 432 leadsto the shell side 442 of the recuperator 402. The duct 432 passesthrough a wall 436 that separates the air 406 from the flue gas 408. Theduct 432 then leads into the shell side 442 of recuperator 402. On theshell side, the air 406 may flow across at 90° to the tubes 422 carryingthe hot flue gas, and the air 406 may flow countercurrent to the tubes422 carrying the flue gas. The shell side 442 leads to the air outlet424 that allows air 406 into combustion chamber 404.

The plate 430 supports one end of the combustion gas tubes 422 at theside where the combustion flue gas enters from the combustion chamber404. The triangular-shaped combustion chamber 404 is defined by theangled wall 418 which reflects radiative heat to a heating plate 410, asshown in FIG. 4. Behind the angled wall 418, a manifold 438 is createdfor the combustion gas to enter one of the plurality of tubes 422. Thecombustion gases may flow from the combustion chamber 404 through anopening created by the upper end of the angled wall 418 and the slantedwall 430. Combustion gases then flow downwardly through the manifold 438and into the plurality of tubes 422. After passing through the tubes422, the hot combustion gases enter a manifold 420 created by the wall436 separating the air in the shell side 442 from the manifold 420. Themanifold 420 directs the combustion gas coming from tubes 422 to theexterior of the recuperator 402 through the flue gas outlet 408.

Referring to FIG. 6, a view of the recuperator 402 is shown, without theangled combustion chamber wall 418. However, in other embodiments, thewall 430 can be the angled combustion chamber wall. In FIG. 6, thecombustion gas tubes 422 are seen more clearly spaced in an array. Thetubes 422 may have fins 426 running longitudinally inside and outside ofthe tubes 422. The outline 428 denotes the flue gas duct leading to theexterior, and is connected to the manifold 420 that receives the flow ofhot flue gas from the exit end of the combustion gas tubes 422.Meanwhile, the incoming air is blocked from entering the flue gasmanifold 438 by the wall 430 separating the combustion gas from theincoming air. The incoming air is therefore behind the wall 430, and thewall 430 directs the incoming air downward eventually leading to the airduct 424 which leads to the combustion chamber 404.

While one embodiment of a recuperator 402 is illustrated and described,it should be apparent to those skilled in the art that various designmodifications may be made to the recuperator 402 to achieve heattransfer between the hot combustion gases and the incoming air used forcombustion.

The V-shaped combustion chamber 404 may be used to burn biomass, liquid,or gas fuel, as described above. Heat is transferred to a heating plate410 through an increase in radiative heat transfer, both through directradiation from the burner to the heating plate 410 and reflected heatfrom the combustion chamber angled walls 418. Heat from the heatingplate 410 is then used to generate electricity by a power conversioncycle. One such device that can be used is the Stirling engine describedabove.

The V-shaped combustion chamber also allows for ash to be collected andremoved from the combustion chamber 404. Additionally, a highertemperature burner, such as for liquid or gas, could be placed in thecombustion chamber 404 to burn liquid fuel with a higher flametemperature than wood. This would further increase the operatingefficiency of the system due to better radiative coupling of the flameto the heating plate 410.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

1. A stove comprising; a combustion chamber; and a heating plate,wherein the combustion chamber includes one or more angled combustionchamber walls, wherein the angle of the one or more combustion chamberwalls is configured to reflect heat produced in the combustion chamberonto the heating plate.
 2. The stove of claim 1, wherein the combustionchamber has a V-shape formed by the combustion chamber angled walls. 3.The stove of claim 1, wherein the one or more angled walls are nothorizontal.
 4. The stove of claim 1, wherein the one or more angledwalls have a lower end toward a center of the combustion chamber and anupper end away from the center of the combustion chamber.
 5. The stoveof claim 1, further comprising one or more air inlet ducts formed from aside of a combustion chamber angled wall.
 6. The stove of claim 1,further comprising combustion gas ducts and air inlet ducts, and arecuperator is located at a base section of the stove, wherein therecuperator thermally couples the combustion gas ducts to the air inletducts.